1. Field of the Invention
This invention relates to a method of effectively erasing the noise developing on a visible image reproduced from a stimulable phosphor sheet carrying a radiation image. This invention particularly relates to a method of effectively erasing the noise developing on such a reproduced visible image due to repeated use of the stimulable phosphor sheet in a radiation image information recording and reproducing method where the stimulable phosphor sheet is exposed to a radiation to record a radiation image therein and then exposed to a stimulating ray which causes it to emit light in the pattern of the stored image, the emitted light is converted to an electric signal, and a visible image corresponding to the radiation image is reproduced by use of the electric signal.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to such radiation as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to a stimulating ray such as visible ray, light is emitted from the phosphor in the pattern of the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 3,859,527 and 4,276,473, U.S. Pat. Application Ser. Nos. 104,855 and 220,780, and Japanese Unexamined Patent Publication No. 56(1981)--11395, it has been proposed to use the stimulable phosphor for recording a radiation image of the human body for medical diagnosis. More specifically, the stimulable phosphor is first exposed to a radiation to have a radiation image stored therein and is then scanned with a stimulating ray which causes it to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor upon stimulation thereof is photoelectrically detected and converted to an electric signal which is processed as desired to reproduce a visible image of a quality suitable for viewing and diagnostic purposes. This radiation image system using a stimulable phosphor has many advantages over conventional radiography using a silver halide photographic material such as that described in U.S. Pat. No. 4,276,473.
In the radiation image recording and reproducing method described above, the final visible image may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube (CRT). The stimulable phosphor sheet used in this method may take any of various forms such as a panel, drum or the like, which are herein generally referred to as sheets. For economical reasons, it is desirable that the stimulable phosphor sheet be used repeatedly in many separate radiographic operations.
In order to reuse the stimulable phosphor sheet, it is necessary for the once-used stimulable phosphor sheet to be made completely free from the previously stored radiation image. Theoretically, the radiation energy of the radiation image stored in the stimulable phosphor sheet should disappear when the sheet is scanned with a stimulating ray of a sufficient intensity to cause it to emit light therefrom in the pattern of the stored radiation image in the course of the radiation image recording and reproducing process as described above. Actually, however, the stored radiation energy is not completely converted to light energy by the stimulating ray used to scan the stimulable phosphor sheet during the aforesaid process. Thus a part of the previously stored radiation image remains in the once-used stimulable phosphor sheet and inconveniently causes noise to occur in the visible image reproduced from the stimulable phosphor sheet when it is reused. For satisfactory results in reusing the stimulable phosphor sheet, the residual radiation image thereon must first be erased completely.
Further, a stimulable phosphor contains a trace of radioactive isotopes such as .sup.226 Ra and .sup.40 K, which emit radiations that the stimulable phosphor sheet stores even when it is not being used in radiography. These traces of radioactive isotopes also constitute a cause for noise in the reproduced visible radiation image. Furthermore, a stimulable phosphor sheet is also affected by environmental radiations such as cosmic rays and X-rays emitted from other X-ray sources and stores the energy thereof. These types of radiation energy (hereinafter referred to as fog) undesirably stored in the stimulable phosphor sheet also causes noise to appear in the visible radiation image reproduced from a reused stimulable phosphor sheet and, therefore, must be erasing before reusing the stimulable phosphor sheet.
In order to avoid the occurrence of noise in the reproduced visible radiation image due to the noise originating from the radiation image previously stored in the stimulable phosphor sheet and due to the fog developing during the storage of the sheet, the applicant has proposed in his U.S. Patent Application Ser. No. 168,795 now U.S. Pat. No. 4,400,619 the stimulable phosphor sheet be stimulated by use of light of wavelengths including the wavelength range of the stimulating ray for the phosphor before storing a radiation image in the stimulable phosphor sheet, thereby to discharge the detrimental radiation energy therefrom to an acceptable extent.
With this method, however, erasing of the residual radiation image and fog in the stimulable phosphor sheet must be effected immediately before using the sheet for radiography. This is necessary to minimize the development of fog in the stimulable phosphor sheet after the erasing is conducted.
The inventors conducted experiments to find the radiation energy levels of the residual image and the fog in the reused phosphor which cause noise in the reproduced visible radiation image to an extent adversely affecting diagnosis. From the results of these experiments, it has been found that, in order to eliminate the detrimental noise due to the residual image, the radiation energy of the radiation image stored in the phosphor must be erased to the order of 10.sup.-4 to 10.sup.-6. Stated differently, the original radiation energy stored in the phosphor must be erased to the level of 0.01 to 0.0001 when the maximum of the original level is 100. On the other hand, the level of the fog developing in the phosphor is generally about 0.1 to 0.001 based on the maximum of the stored original radiation energy which is taken as 100 as described above. It has also been found that the fog must be erased to the level of about 0.01 to 0.0001 in order to prevent the fog from causing detrimental noise in the next radiographic operation.
However, in order to erase the radiation energy of the previously stored radiation image to the order of 10.sup.-4 to 10.sup.-6, the phosphor must be exposed to high illumination for a long length of time, for example to 30,000 lx for 100 to 1,000 seconds using a tungsten-filament lamp. This necessitates a large-scale erasing device and, in addition, erasing must be started a considerable length of time before a radiograph is to be taken. Thus it is very difficult in practical use to carry out such a troublesome erasing operation each time a radiograph is to be taken. Further, it is very inconvenient in practical use to install a large-scale erasing device in the vicinity of the radiographic equipment.